http://www.skyandtelescope.com.au 15WAWE INSPIRINGTheMilkyWayGalaxy
arches across the Chilean sky above the
ruggedlandscapeoftheAtacamaDesert,
site of ESO’s Paranal Observatory.in addition to many other observations,
have revealed that we live in a spiral
galaxywithadiskmorethan100,
light-years across. We now know that
Earth is located more than 25,
light-years from our galaxy’s core and
completes an orbit around it once every
quarter billion years.
But astronomers are still working
tomapourlocalregionofthegalaxy.
The mission of making a precise stellar
measurement catalogue (a science
known asastrometry) moved into space
with the European Space Agency (ESA)
Hipparcos spacecraft, which operated
from1989until1993.ESA’songoing
Gaia mission (see box below) has since
taken up the effort to measure precise
parallaxes for more than one billion
starsintheMilkyWayGalaxy,about
1% of the total galactic population.Beyond the Milky Way
Thenextrungofthecosmicdistance
ladder, the measurement ofCepheid
variable stars, takes us farther out
still. Cepheids brighten and fade in a
methodical pattern. The period with
which a Cepheid varies in brightness
has a direct relationship to its intrinsicGaia: The biggest map of all
The European Space Agency launched the Gaia astrometry
spacecraft on December 18, 2013; the spacecraft became
operational in July 2014. The Gaia mission’s objective is to measure
the positions of about 1 billion stars (that’s about 1 percent of our
galaxy’s stellar population), both in our galaxy and other members
of the Local Group, with an accuracy down to 24 microarcseconds.
Astronomers have been performing spectral and photometric
measurements of all the objects documented by the spacecraft
and using them to derive the space velocities of the Milky Way’s
stars. The enormous dataset will help Gaia scientists build a three-
dimensional structural map of our galaxy.
ESA released the map shown here in September 2016. It shows
an all-sky view of the Milky Way and neighbouring galaxies based
on Gaia’s irst year of observations (July 2014 to September 2015).
Brighter regions indicate a denser concentration of stars. The dark
regions across the galactic plane correspond to light-obscuring
clouds of interstellar gas and dust.(not perceived) luminosity, making
these stars great standard candles:
Find a Cepheid in a cluster or galaxy,
measure how bright it appears to be,
and you can determine how far away it
is, because brightness drops off by the
inverse square of the source’s distance.
Henrietta Swan Leavitt first noticed
this crucial relationship in 1908 while
examining variable stars in the Large
Magellanic Cloud.
Edwin Hubble’s discovery of a
Cepheid in the Andromeda Galaxy
in 1925 allowed him to calculate the
galaxy’s distance — 1.5 million light-
years. Not a bad first estimate, though
short of today’s value of 2.54 million
light-years. Hubble’s work settled a
debate in astronomy by showing that
many of the objects we observe lie far
beyond the Milky Way Galaxy.
Think about it: Less than one
century ago, many astronomy texts
stated that the Milky Way constituted
the entire breadth of the universe, with
star charts labelling enigmatic fuzzy
patches with names like the Andromeda
“Nebula”. We now think there are more
than a trillion galaxies out there, many
of which are much larger than ours.Going with the flow
The discovery of cosmic redshift, our
next stop on the cosmic distance
ladder, gave us another yardstick toESA / GAIA / DPAC
measure even larger distances on extra-
galactic scales. Redshift is the shift in a
source’s spectral lines to longer, redder
wavelengths, similar to the change in
pitch due to the Doppler effect as a
train’s whistle recedes from the listener.
Redshift exists because the source of
light is moving with respect to our
location. Conversely, an object moving
towards us exhibits blueshift in its
spectral lines.
Vesto Slipher made the first
measurements of blueshift in 1912, of
the Andromeda Galaxy. Thanks to the
shift in its spectral lines, he discovered
that the galaxy was moving toward us
at an astounding velocity and probably
wasn’t in the Milky Way at all. Since
then, we’ve been able to use the Doppler
shift, standard candles, and other
methods to determine the distances
to nearby galaxies and how they move.
This work reveals that the Milky Way,
Andromeda and more than 50 known
galaxies compose a small cluster we
call the Local Group. In turn, the Local
Group belongs to the Virgo Supercluster,
which contains over 100 galaxy groups
in a 100-million-light-year span. Virgo
itself belongs to the great Laniakea
Supercluster of galaxy clusters, which
holds over 100,000 galaxies.
Census studies of galaxies begun in
the 1970s point towards an additional
flow of galaxy clusters through the